/**
 * Copyright 1996-2014 FoxBPM ORG.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * 
 * @author kenshin
 * @author ych
 */
package org.foxbpm.engine.impl.util;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;

import sun.misc.ServiceConfigurationError;

/**
 * A simple service-provider loading facility.
 * 
 * <p>
 * A <i>service</i> is a well-known set of interfaces and (usually abstract)
 * classes. A <i>service provider</i> is a specific implementation of a service.
 * The classes in a provider typically implement the interfaces and subclass the
 * classes defined in the service itself. Service providers can be installed in
 * an implementation of the Java platform in the form of extensions, that is,
 * jar files placed into any of the usual extension directories. Providers can
 * also be made available by adding them to the application's class path or by
 * some other platform-specific means.
 * 
 * <p>
 * For the purpose of loading, a service is represented by a single type, that
 * is, a single interface or abstract class. (A concrete class can be used, but
 * this is not recommended.) A provider of a given service contains one or more
 * concrete classes that extend this <i>service type</i> with data and code
 * specific to the provider. The <i>provider class</i> is typically not the
 * entire provider itself but rather a proxy which contains enough information
 * to decide whether the provider is able to satisfy a particular request
 * together with code that can create the actual provider on demand. The details
 * of provider classes tend to be highly service-specific; no single class or
 * interface could possibly unify them, so no such type is defined here. The
 * only requirement enforced by this facility is that provider classes must have
 * a zero-argument constructor so that they can be instantiated during loading.
 * 
 * <p>
 * <a name="format"> A service provider is identified by placing a
 * <i>provider-configuration file</i> in the resource directory
 * <tt>META-INF/services</tt>. The file's name is the fully-qualified <a
 * href="../lang/ClassLoader.html#name">binary name</a> of the service's type.
 * The file contains a list of fully-qualified binary names of concrete provider
 * classes, one per line. Space and tab characters surrounding each name, as
 * well as blank lines, are ignored. The comment character is <tt>'#'</tt> (
 * <tt>'&#92;u0023'</tt>, <font size="-1">NUMBER SIGN</font>); on each line all
 * characters following the first comment character are ignored. The file must
 * be encoded in UTF-8.
 * 
 * <p>
 * If a particular concrete provider class is named in more than one
 * configuration file, or is named in the same configuration file more than
 * once, then the duplicates are ignored. The configuration file naming a
 * particular provider need not be in the same jar file or other distribution
 * unit as the provider itself. The provider must be accessible from the same
 * class loader that was initially queried to locate the configuration file;
 * note that this is not necessarily the class loader from which the file was
 * actually loaded.
 * 
 * <p>
 * Providers are located and instantiated lazily, that is, on demand. A service
 * loader maintains a cache of the providers that have been loaded so far. Each
 * invocation of the {@link #iterator iterator} method returns an iterator that
 * first yields all of the elements of the cache, in instantiation order, and
 * then lazily locates and instantiates any remaining providers, adding each one
 * to the cache in turn. The cache can be cleared via the {@link #reload reload}
 * method.
 * 
 * <p>
 * Service loaders always execute in the security context of the caller. Trusted
 * system code should typically invoke the methods in this class, and the
 * methods of the iterators which they return, from within a privileged security
 * context.
 * 
 * <p>
 * Instances of this class are not safe for use by multiple concurrent threads.
 * 
 * <p>
 * Unless otherwise specified, passing a <tt>null</tt> argument to any method in
 * this class will cause a {@link NullPointerException} to be thrown.
 * 
 * 
 * <p>
 * <span style="font-weight: bold; padding-right: 1em">Example</span> Suppose we
 * have a service type <tt>com.example.CodecSet</tt> which is intended to
 * represent sets of encoder/decoder pairs for some protocol. In this case it is
 * an abstract class with two abstract methods:
 * 
 * <blockquote>
 * 
 * <pre>
 * public abstract Encoder getEncoder(String encodingName);
 * 
 * public abstract Decoder getDecoder(String encodingName);
 * </pre>
 * 
 * </blockquote>
 * 
 * Each method returns an appropriate object or <tt>null</tt> if the provider
 * does not support the given encoding. Typical providers support more than one
 * encoding.
 * 
 * <p>
 * If <tt>com.example.impl.StandardCodecs</tt> is an implementation of the
 * <tt>CodecSet</tt> service then its jar file also contains a file named
 * 
 * <blockquote>
 * 
 * <pre>
 * META - INF / services / com.example.CodecSet
 * </pre>
 * 
 * </blockquote>
 * 
 * <p>
 * This file contains the single line:
 * 
 * <blockquote>
 * 
 * <pre>
 * com.example.impl.StandardCodecs    # Standard codecs
 * </pre>
 * 
 * </blockquote>
 * 
 * <p>
 * The <tt>CodecSet</tt> class creates and saves a single service instance at
 * initialization:
 * 
 * <blockquote>
 * 
 * <pre>
 * private static ServiceLoader&lt;CodecSet&gt; codecSetLoader = ServiceLoader.load(CodecSet.class);
 * </pre>
 * 
 * </blockquote>
 * 
 * <p>
 * To locate an encoder for a given encoding name it defines a static factory
 * method which iterates through the known and available providers, returning
 * only when it has located a suitable encoder or has run out of providers.
 * 
 * <blockquote>
 * 
 * <pre>
 * public static Encoder getEncoder(String encodingName) {
 * 	for (CodecSet cp : codecSetLoader) {
 * 		Encoder enc = cp.getEncoder(encodingName);
 * 		if (enc != null)
 * 			return enc;
 * 	}
 * 	return null;
 * }
 * </pre>
 * 
 * </blockquote>
 * 
 * <p>
 * A <tt>getDecoder</tt> method is defined similarly.
 * 
 * 
 * <p>
 * <span style="font-weight: bold; padding-right: 1em">Usage Note</span> If the
 * class path of a class loader that is used for provider loading includes
 * remote network URLs then those URLs will be dereferenced in the process of
 * searching for provider-configuration files.
 * 
 * <p>
 * This activity is normal, although it may cause puzzling entries to be created
 * in web-server logs. If a web server is not configured correctly, however,
 * then this activity may cause the provider-loading algorithm to fail
 * spuriously.
 * 
 * <p>
 * A web server should return an HTTP 404 (Not Found) response when a requested
 * resource does not exist. Sometimes, however, web servers are erroneously
 * configured to return an HTTP 200 (OK) response along with a helpful HTML
 * error page in such cases. This will cause a {@link ServiceConfigurationError}
 * to be thrown when this class attempts to parse the HTML page as a
 * provider-configuration file. The best solution to this problem is to fix the
 * misconfigured web server to return the correct response code (HTTP 404) along
 * with the HTML error page.
 * 
 * @param <S>
 *            The type of the service to be loaded by this loader
 * 
 * @author Mark Reinhold
 * @version 1.10, 06/04/10
 * @since 1.6
 * 
 *        从1.6JDK中拷出，主要为了foxbpm引擎支持1.5版本JDK
 */

@SuppressWarnings("restriction")
public final class ServiceLoader<S> implements Iterable<S> {

	private static final String PREFIX = "META-INF/services/";

	// The class or interface representing the service being loaded
	private Class<S> service;

	// The class loader used to locate, load, and instantiate providers
	private ClassLoader loader;

	// Cached providers, in instantiation order
	private LinkedHashMap<String, S> providers = new LinkedHashMap<String, S>();

	// The current lazy-lookup iterator
	private LazyIterator lookupIterator;

	/**
	 * Clear this loader's provider cache so that all providers will be
	 * reloaded.
	 * 
	 * <p>
	 * After invoking this method, subsequent invocations of the
	 * {@link #iterator() iterator} method will lazily look up and instantiate
	 * providers from scratch, just as is done by a newly-created loader.
	 * 
	 * <p>
	 * This method is intended for use in situations in which new providers can
	 * be installed into a running Java virtual machine.
	 */
	public void reload() {
		providers.clear();
		lookupIterator = new LazyIterator(service, loader);
	}

	private ServiceLoader(Class<S> svc, ClassLoader cl) {
		service = svc;
		loader = cl;
		reload();
	}

	@SuppressWarnings("rawtypes")
	private static void fail(Class service, String msg, Throwable cause) throws ServiceConfigurationError {
		throw new ServiceConfigurationError(cause);
	}

	@SuppressWarnings("rawtypes")
	private static void fail(Class service, String msg) throws ServiceConfigurationError {
		throw new ServiceConfigurationError(service.getName() + ": " + msg);
	}

	@SuppressWarnings("rawtypes")
	private static void fail(Class service, URL u, int line, String msg) throws ServiceConfigurationError {
		fail(service, u + ":" + line + ": " + msg);
	}

	// Parse a single line from the given configuration file, adding the name
	// on the line to the names list.
	//
	@SuppressWarnings("rawtypes")
	private int parseLine(Class service, URL u, BufferedReader r, int lc, List<String> names) throws IOException, ServiceConfigurationError {
		String ln = r.readLine();
		if (ln == null) {
			return -1;
		}
		int ci = ln.indexOf('#');
		if (ci >= 0)
			ln = ln.substring(0, ci);
		ln = ln.trim();
		int n = ln.length();
		if (n != 0) {
			if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))
				fail(service, u, lc, "Illegal configuration-file syntax");
			int cp = ln.codePointAt(0);
			if (!Character.isJavaIdentifierStart(cp))
				fail(service, u, lc, "Illegal provider-class name: " + ln);
			for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
				cp = ln.codePointAt(i);
				if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))
					fail(service, u, lc, "Illegal provider-class name: " + ln);
			}
			if (!providers.containsKey(ln) && !names.contains(ln))
				names.add(ln);
		}
		return lc + 1;
	}

	// Parse the content of the given URL as a provider-configuration file.
	//
	// @param service
	// The service type for which providers are being sought;
	// used to construct error detail strings
	//
	// @param u
	// The URL naming the configuration file to be parsed
	//
	// @return A (possibly empty) iterator that will yield the provider-class
	// names in the given configuration file that are not yet members
	// of the returned set
	//
	// @throws ServiceConfigurationError
	// If an I/O error occurs while reading from the given URL, or
	// if a configuration-file format error is detected
	//
	@SuppressWarnings("rawtypes")
	private Iterator<String> parse(Class service, URL u) throws ServiceConfigurationError {
		InputStream in = null;
		BufferedReader r = null;
		ArrayList<String> names = new ArrayList<String>();
		try {
			in = u.openStream();
			r = new BufferedReader(new InputStreamReader(in, "utf-8"));
			int lc = 1;
			while ((lc = parseLine(service, u, r, lc, names)) >= 0)
				;
		} catch (IOException x) {
			fail(service, "Error reading configuration file", x);
		} finally {
			try {
				if (r != null)
					r.close();
				if (in != null)
					in.close();
			} catch (IOException y) {
				fail(service, "Error closing configuration file", y);
			}
		}
		return names.iterator();
	}

	// Private inner class implementing fully-lazy provider lookup
	//
	private class LazyIterator implements Iterator<S> {

		Class<S> service;
		ClassLoader loader;
		Enumeration<URL> configs = null;
		Iterator<String> pending = null;
		String nextName = null;

		private LazyIterator(Class<S> service, ClassLoader loader) {
			this.service = service;
			this.loader = loader;
		}

		public boolean hasNext() {
			if (nextName != null) {
				return true;
			}
			if (configs == null) {
				try {
					String fullName = PREFIX + service.getName();
					if (loader == null)
						configs = ClassLoader.getSystemResources(fullName);
					else
						configs = loader.getResources(fullName);
				} catch (IOException x) {
					fail(service, "Error locating configuration files", x);
				}
			}
			while ((pending == null) || !pending.hasNext()) {
				if (!configs.hasMoreElements()) {
					return false;
				}
				pending = parse(service, configs.nextElement());
			}
			nextName = pending.next();
			return true;
		}

		public S next() {
			if (!hasNext()) {
				throw new NoSuchElementException();
			}
			String cn = nextName;
			nextName = null;
			try {
				S p = service.cast(Class.forName(cn, true, loader).newInstance());
				providers.put(cn, p);
				return p;
			} catch (ClassNotFoundException x) {
				fail(service, "Provider " + cn + " not found");
			} catch (Throwable x) {
				fail(service, "Provider " + cn + " could not be instantiated: " + x, x);
			}
			throw new Error(); // This cannot happen
		}

		public void remove() {
			throw new UnsupportedOperationException();
		}

	}

	/**
	 * Lazily loads the available providers of this loader's service.
	 * 
	 * <p>
	 * The iterator returned by this method first yields all of the elements of
	 * the provider cache, in instantiation order. It then lazily loads and
	 * instantiates any remaining providers, adding each one to the cache in
	 * turn.
	 * 
	 * <p>
	 * To achieve laziness the actual work of parsing the available
	 * provider-configuration files and instantiating providers must be done by
	 * the iterator itself. Its {@link java.util.Iterator#hasNext hasNext} and
	 * {@link java.util.Iterator#next next} methods can therefore throw a
	 * {@link ServiceConfigurationError} if a provider-configuration file
	 * violates the specified format, or if it names a provider class that
	 * cannot be found and instantiated, or if the result of instantiating the
	 * class is not assignable to the service type, or if any other kind of
	 * exception or error is thrown as the next provider is located and
	 * instantiated. To write robust code it is only necessary to catch
	 * {@link ServiceConfigurationError} when using a service iterator.
	 * 
	 * <p>
	 * If such an error is thrown then subsequent invocations of the iterator
	 * will make a best effort to locate and instantiate the next available
	 * provider, but in general such recovery cannot be guaranteed.
	 * 
	 * <blockquote style="font-size: smaller; line-height: 1.2"><span
	 * style="padding-right: 1em; font-weight: bold">Design Note</span> Throwing
	 * an error in these cases may seem extreme. The rationale for this behavior
	 * is that a malformed provider-configuration file, like a malformed class
	 * file, indicates a serious problem with the way the Java virtual machine
	 * is configured or is being used. As such it is preferable to throw an
	 * error rather than try to recover or, even worse, fail
	 * silently.</blockquote>
	 * 
	 * <p>
	 * The iterator returned by this method does not support removal. Invoking
	 * its {@link java.util.Iterator#remove() remove} method will cause an
	 * {@link UnsupportedOperationException} to be thrown.
	 * 
	 * @return An iterator that lazily loads providers for this loader's service
	 */
	public Iterator<S> iterator() {
		return new Iterator<S>() {

			Iterator<Map.Entry<String, S>> knownProviders = providers.entrySet().iterator();

			public boolean hasNext() {
				if (knownProviders.hasNext())
					return true;
				return lookupIterator.hasNext();
			}

			public S next() {
				if (knownProviders.hasNext())
					return knownProviders.next().getValue();
				return lookupIterator.next();
			}

			public void remove() {
				throw new UnsupportedOperationException();
			}

		};
	}

	/**
	 * Creates a new service loader for the given service type and class loader.
	 * 
	 * @param service
	 *            The interface or abstract class representing the service
	 * 
	 * @param loader
	 *            The class loader to be used to load provider-configuration
	 *            files and provider classes, or <tt>null</tt> if the system
	 *            class loader (or, failing that, the bootstrap class loader) is
	 *            to be used
	 * 
	 * @return A new service loader
	 */
	public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader) {
		return new ServiceLoader<S>(service, loader);
	}

	/**
	 * Creates a new service loader for the given service type, using the
	 * current thread's {@linkplain java.lang.Thread#getContextClassLoader
	 * context class loader}.
	 * 
	 * <p>
	 * An invocation of this convenience method of the form
	 * 
	 * <blockquote>
	 * 
	 * <pre>
	 * ServiceLoader.load(<i>service</i>)
	 * </pre>
	 * 
	 * </blockquote>
	 * 
	 * is equivalent to
	 * 
	 * <blockquote>
	 * 
	 * <pre>
	 * ServiceLoader.load(<i>service</i>,
	 *                    Thread.currentThread().getContextClassLoader())
	 * </pre>
	 * 
	 * </blockquote>
	 * 
	 * @param service
	 *            The interface or abstract class representing the service
	 * 
	 * @return A new service loader
	 */
	public static <S> ServiceLoader<S> load(Class<S> service) {
		ClassLoader cl = Thread.currentThread().getContextClassLoader();
		return ServiceLoader.load(service, cl);
	}

	/**
	 * Creates a new service loader for the given service type, using the
	 * extension class loader.
	 * 
	 * <p>
	 * This convenience method simply locates the extension class loader, call
	 * it <tt><i>extClassLoader</i></tt>, and then returns
	 * 
	 * <blockquote>
	 * 
	 * <pre>
	 * ServiceLoader.load(<i>service</i>, <i>extClassLoader</i>)
	 * </pre>
	 * 
	 * </blockquote>
	 * 
	 * <p>
	 * If the extension class loader cannot be found then the system class
	 * loader is used; if there is no system class loader then the bootstrap
	 * class loader is used.
	 * 
	 * <p>
	 * This method is intended for use when only installed providers are
	 * desired. The resulting service will only find and load providers that
	 * have been installed into the current Java virtual machine; providers on
	 * the application's class path will be ignored.
	 * 
	 * @param service
	 *            The interface or abstract class representing the service
	 * 
	 * @return A new service loader
	 */
	public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {
		ClassLoader cl = ClassLoader.getSystemClassLoader();
		ClassLoader prev = null;
		while (cl != null) {
			prev = cl;
			cl = cl.getParent();
		}
		return ServiceLoader.load(service, prev);
	}

	/**
	 * Returns a string describing this service.
	 * 
	 * @return A descriptive string
	 */
	public String toString() {
		return "java.util.ServiceLoader[" + service.getName() + "]";
	}

}
